Really?  Last time I checked ETH has quite a lot of nodes.  We can't get a 100% accurate estimate of how many Bitcoin full nodes there are, because sites like this are only talking about full nodes with open ports.

A lack of full nodes is a problem because of the importance of decentralisation and removing the control of entities such as banks.  However, fewer full nodes would not result in less scalability - Bitcoin and ETH are just not scaled properly, which does not relate to the exact node count or the exact amount of hashrate.  These things are about security, not scalability.